Biosand filters (BSF) are a simple water treatment method which was developed from slow sand filters, and can remove pathogens and suspended solids from water.
Mechanism of Treatment
Water is treated through a combination of biological and physical processes:
Mechanical trapping – suspended solids an pathogens are physically trapped in the spaces between the sand grains
Predation – Micro-organisms in the biolayer (“schmetzdecke”) eat some pathogens
Adsorption – Some pathogens get stuck to the sand grains
Natural death – Some pathogens die because they lack food or oxygen, or because they finish their life cycle and die of old age.
- Can be built locally using available materials and labor
- Easy to operate
- Relatively inexpensive
- Can be used with most water sources (well, borehole, pond, river, tap-stand, rain)
- Does not remove dissolved contaminants or chemicals (e.g. salt, arsenic, fluoride)
- Can’t be used with chlorinated water, as chlorine will kill the biolayer
- Up to 100% helminthes (worms)
- Up to 100% protozoa
- Up to 98.5% of bacteria
- 70-90% of viruses
- Up to 95% of turbidity
Chlorine Dioxide Water Treatment
Hydrogen Peroxide Water Treatment
Ozone Water Purification
UV Water Disinfection
Reverse Osmosis (RO) is a water treatment method that uses filtration to remove particles from water based on size.
Mechanism of Treatment
Osmosis is the scientific term to describe the natural movement of fluid across a membrane (filter) from a lower concentrated solution to a higher concentrated solution, which will then weaken (dilute) the higher concentrate.
Reverse osmosis is primarily a filtration/straining method, and is sometimes called hyper-filtration. Reverse osmosis occurs when pressure is applied to the higher concentrate side, forcing the fluid to move through the membrane (filter) in the opposite direction that it would naturally flow. The result is that fluid moves to the lower concentrate side, but the contaminants cannot pass through the filter, so they are left on the higher concentrate side.
Particle filtration – removes particles of 3.9 x 10 -5 inches or larger
Microfiltration – removes parties of 50nm or larger
Ultrafiltration – removes particles of 3nm or larger
Nanofiltration – removes particles of 1nm or larger
Reverse osmosis / hyperfiltration – removes particles of .01 nm or larger
- RO equipment is standardized(pumps, motors, valves, flow meters, pressure gauges etc.) so the learning curve of unskilled labor is short
- Can remove all contaminants that are smaller in size than a water molecule (arsenic, sodium, copper, lead, some organic chemicals, fluoride, bacteria and pathogens)
- Compact system, requires less space than some systems like distillation.
- A good solution when having to deal with high TDS (Total dissolved solids) or high sodium (Salt) content.
- Usually needs to be used in combination with other treatment methods, such as a pre-filter (to remove large contaminants), a carbon filter (used before RO to trap organic chemicals and remove chlorine which can damage the RO membrane filters, and can be used again after RO to capture chemicals not removed by RO), or ultraviolet (to sterilize microbes)
- Can only remove contaminants that are smaller in size than a water molecule – does not remove VOC’s (volatile organic chemicals), chlorine and chloramines, pharmaceuticals, pesticides.
- Healthy, naturally occurring minerals are also removed.
- The membrane must be cleaned often, and can trap bacteria which can potentially pose other problems if membrane is compromised.
- It requires diligent maintenance and has an ongoing cost of filter replacement (pre-filters every year, RO filters every 2-3 years)
- Requires electricity to run the pumps (infrastructure)
- Has a significant initial cost to purchase.
- Loses at least 1 gallon of water per 1 gallon of clean water produced.
Up to 99% of dissolved salts, particles, colloids, organics, bacteria, and pyrogens